For many years, external optical modulators have been made out of electro-optic material, such as lithium niobate. Optical waveguides are formed within the electro-optic material, with metal contact regions disposed on the surface of each waveguide arm. The application of a voltage to a metal contact will modify the refractive index of the waveguide region underneath the contact, thus changing the speed of propagation along the waveguide. By applying the voltage(s) that produce a π phase shift between the two arms, a nonlinear (digital) Mach-Zehnder modulator is formed. In particular, the optical signal is launched into the waveguide, and the electrical signal input is applied to the contacts (using proper voltage levels, as mentioned above). The optical signal is phase modulated as it propagates along the arms to generate the output optical signal. A similar result is possible with a linear (analog) optical output signal.
Although this type of external modulator has proven extremely useful, there is an increasing desire to form various optical components, subsystems and systems on silicon-based platforms. It is further desirable to integrate the various electronic components associated with such systems (for example, the input electrical data drive circuit for an electro-optic modulator) with the optical components on the same silicon substrate. Clearly, the use of lithium niobate-based optical devices in such a situation is not an option. Various other conventional electro-optic devices are similarly of a material (such as Ill-V compounds) that are not directly compatible with a silicon platform.
A significant advance in the ability to provide optical modulation in a silicon-based platform has been made, as disclosed in U.S. Pat. No. 6,845,198 issued to R. K. Montgomery et al. on Jan. 18, 2005 and assigned to the assignee of this application. FIG. 1 illustrates one exemplary arrangement of a silicon-based modulator device as disclosed in (for example) the Montgomery et al. patent. In this case, SOI-based optical modulator 1 comprises a doped silicon layer 2 (typically, polysilicon) disposed in an overlapped arrangement with an oppositely-doped portion of a sub-micron thick silicon surface layer 4 (often referred to in the art as an SOI layer). SOI layer 4 is shown as the surface layer of a conventional SOI structure 5, which further includes a silicon substrate 6 and buried oxide layer 7. Importantly, a relatively thin dielectric layer 8 (such as, for example, silicon dioxide, silicon nitride or the like) is disposed along the overlapped region between SOI layer 4 and doped silicon layer 2. The overlapped area defined by silicon layer 2, dielectric 8 and SOI layer 4 defines the ‘active region’ of optical modulator 1. Free carriers will accumulate and deplete on either side of dielectric 8 as a function of the voltages applied to SOI layer 4 (VREF4) and/or doped silicon layer 2 (VREF2). The modulation of the free carrier concentration results in changing the effective refractive index in the active region, thus introducing phase modulation of an optical signal propagating along a waveguide defined by the action region (the optical signal propagating along the y-axis, in the direction perpendicular to the paper).
A remaining area of concern for optical modulators of the type developed by Montgomery et al. is associated with the ability to modify/tune the modulation in order to adjust/correct for process variations, changes in ambient conditions (such as temperature), device aging, and the like.